Rationalizing theSuperior Catalytic Efficiency ofNickel Nitride vs Nickel Sulfide for Alkaline Hydrogen Evolution Reactionfrom Bubble Dynamics Study and Density Functional Theory (DFT) Calculations

Due to their high electronic conductivity, high catalytic activity, and superior chemical stability, nickel nitrides and sulfides have been demonstrated to be cost-effective and robust electrocatalysts for achieving HER under alkaline conditions. Herein, we report on a simple and optimized approach to directly grow single-phase nickel nitride (Ni₃N) and nickel sulfide (Ni₃S₂) on a flat nickel substrate. Different preparation conditions are tested in order to achieve materials exhibiting the best electrocatalytic efficiency for HER. The optimized Ni₃N and Ni₃S₂ on nickel are obtained at 700 °C for 30 min under ammonia gas flow and at 350 °C for 1 h in 10% H₂S/H₂ gas mixture, respectively. Ni₃N operate HER in 1 M KOH more efficiently than Ni₃S₂, as supported by overpotential values of 0.189, 0.291, and 0.342 V measured at 10, 100, and 200 mA cm^–2, respectively, which are lower than those measured for Ni₃S₂, i.e., 0.204, 0.351, and 0.417 V. Remarkably, it is worth noticing that its HER activity competes with that of Pt for current densities higher than 200 mA cm^–2. Its superior catalytic activity is corroborated by additional cyclic voltammetry (Tafel slopes) and electrochemical impedance spectroscopy measurements. Moreover, Ni₃N and Ni₃S₂ are found to be stable over 45 h of electrolysis at 10 mA cm^–2 with a potential change of only 24 and 31 mV, respectively. To gain further understanding on the electrocatalytic HER activities of both materials, density functional theory (DFT) calculations and an in situ bubble dynamics study are performed. Owing to the more hydrophilic character of Ni₃N, smaller H₂ bubbles form and detach more rapidly from the surface which leads to a fast renewal of the active surface for HER. In contrast, for Ni₃S₂, both the bubble size and the retention time increase, leading to the adverse blockage of the active sites and requiring higher overpotential for HER. This observation perfectly aligns with DFT calculations, which show that H₂O adsorption is predominantly favored on the Ni₃N surface. Our work highlights that the use of a planar electrocatalyst support instead of a foam-type porous one is essential to evaluate the alone contribution of the electrocatalyst on the electrogenerated gas bubble dynamics and consequently the impact on the catalytic performance.